EEG additional questions and answers PDF

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EEG additional questions and answers...

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Answer the following questions and submit to your TA before leaving lab. Additional Observations 1. What is the approximate peak-to-peak amplitude of the EEG signal? A: 50 microvolts 2. Look at the rate/time display (the dropdown box in the upper right with a time value next to it). What is the sampling rate of your data? A: 400 per second Additional Experiments and Questions for Thought: You are not expected to answer all of these questions correctly. Please work independently and answer the questions to the best of your ability. Questions will be considered correct if you have answered them thoughtfully and completely. 1. Open the input amplifier dialog box. To open it, click the Channel Function pop-up menu (the drop box on the right side of the channel) of the EEG channel, and choose the “input amplifier” command. Turn off the Mains Filter (60 Hz notch filter). Can you see a difference in your recording? In your spectrogram? If so, describe the difference and its cause. If not, why isn’t there a difference? A: No I did not see a difference in recording because the lights were off and thus, the EEG leads were not able to pick up current from other sources. 2. With the Mains Filter off, record EEG signals with the 3 electrode leads in two conditions: 1. braided and 2. unbraided and spread to form the largest loop possible. Can you see a difference in your recording? In your spectrogram? If so, describe the difference and its cause. If not, why isn’t there a difference? A: Braiding the leads resulted in a slight decrease of magnitude of our recording because braiding reduces the size of the inductive loop. Spreading the leads increased the size of the inductive loop and resulted in a slight increase of magnitude.

3. Why did you abrade the skin? A: Abrading the skin removes dead skin cells that would have raised the impedance between electrode and skin. Thus, abrading the skin reduces the impedance between the electrode and the skin. 4. In the experiment, you used 3 electrodes: positive, negative and ground. Given these 3 inputs, how was the signal calculated? A: The amplifier measured a differential measurement. It measured the positive signal to ground and the negative signal to ground and found the difference in the two signals. EEG = (pos – ground) – (neg – ground). However, the ground in this case is not earth ground, rather it is an isolated ground for the measurement circuit but it is not connected to ground for wall plug. Patients are never connected to earth ground because a malfunction in the equipment could potentially expose the patient to the current from the mains and lead to electrical shock or electrocution What is the purpose of each of the electrodes? A: The purpose and advantage of the electrodes is that the ground electrode will be exposed to the same room noise as the positive and negative electrodes. The room noise measured on the ground electrode is then subtracted from the recorded signal rather than just measuring the difference between the positive and negative electrodes directly. 5. In order to record the signal, the analog signal must be digitally sampled at the sampling rate specified. Sampling is accomplished with a digital circuit. Do you think the circuit samples the signal at its recorded magnitude or at an amplified magnitude? Why or why not?

A: The circuit would have amplified the signal before sampling the data. The sampling is accomplished with an analog to digital (A/D) converter that is a TTL circuit device. It probably samples signals within a range of +/- 5 V or +/- 15 V. The recorded signal would have been amplified to fill that range before sampling the data.

6. Why do you think the EEG waveshape changed when the eyes were closed? A: When the eyes were open, the eye muscles and retina recorded electrical activity that was picked up by the EEG. Thus, closing the eyes reduced the interference and calmed the EEG waveshape. 7. One way of reducing noise in biological signal data is by averaging the data. The figure below shows data recorded from a nerve in the arm.

The initial negative signal that goes off the scale of the recording is the response to a stimulator. The following positive and negative waves result from the cumulative measurement of many resulting action potentials in the neurons of the nerve. To reduce the noise in the data, the signal would be recorded many times (typically 100) and the resulting signals averaged together. The biological potential remains constant and is not altered by averaging, but the noise randomly varies and is removed by the averaging process. Could you use averaging to reduce the noise in an EEG signal? Why or why not? A: Because the EEG waves are not temporarily synchronized, you could not average an EEG. The averaging technique works for the neural stimulator because the stimulus always synchronizes the action potentials. The action potentials in the nerves all occur in response to the stimulus with a predictable delay. When you average the signals, you start your recording at a constant time with reference to the stimulus....